Pressurized centrifuge



July 13, 1965 R. A. KoFFlNKl-z ETAL 3,194,492

PRES SURI ZED CENTRIFUGE 2 Sheets--Sheetl l Filed June 28, 1962 July 13,1965 R. A. KOFFINKE l-:TAL 3,194,492

PRESSURI ZED CENTRIFUGE Filed June 28, 1962 2 Sheets-Sheet 2 jj j UnitedStates Patent O 3,194,492 llllSSURlZED CENTREFUGE Richard A. Kofnke, 638Comm-on St., Walpole, Mass., and Horace l?. Cools, 33 Leonard St.,Foxboro, Mass. Filed .lune 28, 1962, Ser. No. 205,937 6 Claims. (Cl.233-7) This invention relates to centrifugal separators for separatingsolids from liquids to produce dry solids or to clarify liquid, and moreparticularly to the provision of pressurized separator constructionscapable of separating solid particles of sizes ranging from 1A; inchlong and larger down to the micron ran-ge continuously for an extendedperiod.

Many processes for producing pharmaceuticals, food products, syntheticresins, metals, etc., are greatly improved and some absolutely requireconducting a solidsliquid .separation under high pressure, by which Wemean pressures above psi. gauge, ranging upwardly of 75 and 150 p.s.i.gauge. Such high pressures prevent certain materials dissolved inliquids from precipitating to contaminate the soli-ds and also enableheating of the liquid to obtain the low slurry vi'scosities needed forcentrifugal separation.

It is very desirable that any centrifugal separator construction beuniversal, suitable without major alteration, for a Wide range of theseprocesses and that it be capable Iof operation over an extended periodwithout repairs on the order of 10,000 hours and more. Otherwise, thecosts of construction 4and maintaining such machines is greatlyincreased.

A completely satisfactory pressurized separator has not been producedbecause of the unique sealing and leakage problems that occur at thepoint where the separator driving shaft passes between the yatmosphereand the pressurized casing. Solid particles are always dispersed in thepressurized processing iluids within the casing and any outward leakagewould cause these particles to deposit in the .seals and cause ythem towear out quickly. Furtheromore, processing gases are often explosive,noxious or very expensive, and their loss to the atmosphere cannot betolerated. On the other hand, leakage of sealing liquids into the casingfrom a buffer seal chamber cannot. be tolerated where the liquids areincompatable L with the process, and in many instances such liquids thatare compatible are also noxious, volatile, or expensive, which makes theuse of liquid as a sealant unsatisfactory for a universal pressurizedseparator.

Small quantities of dry inert gas such as nitrogen, or argon can betolerated by almost all processes, but resort to such gas as a sealingfluid in a seal chamber has appeared to raise more problems than itsolves. Gas is extremely difficult to seal against excessive leakageinto the process and to the atmosphere, and it has lbeeny consideredprohibitively expensive to provide a seal construction which hasuniversal application and can last the thou-sands of hours required incontinuously operating separators. Labyrinth and other clearance sealsare out of the question because of excessive leakage. In contact sealsfor dry gas (by which we mean seals which maintain low friction contactbetween a member carried by a shaft and a stationary member fixed orsealed to the casing for primary sealing), if the relatively movingparts are pressed too tightly together, since there is no liquidlubrication,

a the seal will be destroyed within a few hundred hours but if held `toolightly, the part-s will allow unduly large quantities `of gas to passinto the separat-or casing, which wastes the expensive gas, andsometimes can disruptthe process or require special equipment to remove.For contact seals, the primary sealing surfaces must therefore ice besubjected to a critical light loading which appeared impractical for thelong life requirements of numerous processes in which centrifugalseparators are employed. Furthermore, the ordinary face contact sealsfor gas are extremely expensive to manufacture.

In `addition to the foregoing problems which apply to any pressurizedcentrifuge, there have been further unsolved problem-s with regard tothe solid walled bowl, internal conveyer type centrifuges which ,areespecially preferred where it is` necessary to continuously producevirtually dry solids. One of these problems is that such centrifugesrequire two horizontal, concentric high-speed trunnions opera-ting atdifferent speeds, the vouter one carrying the bowl, and the inner onecarrying a screw conveyer. Here the sealing problem is not merely toseal the outside of the bowl trunnion to the casing, but also to insurethat the process gas and solids do not escape between the trunni-ons.Also, with such centrifuges, :any seal that is employed should be ofsmall size as the length and' mass of the moving elements not -onlycontribute to the expense of the machine but also limit the speed atwhich it can operate.

These solid bowl continuous centrifuges in :small sizes, e.g. 10 gallonsper minute rating or less, are preferably -of the overhung type in whichthe bowl and conveyer extend as a `cantilever intol the casing,supported' from only one end. In such constructions casing gas pressurewill impose a substantial axial thrust upon the horizontal trunnion,which had been thought to dictate a complete redesign of prior machinesto ena-ble high pressure operation, to incorporate main thrust bearings,which would add greatly to the expense of such centrifuges.

The principal object of the present invention is to surmount or avoidall of these problems.

The invention provides a means for adapting centrifugal separators ofthe various b-asic designs so that they can be employed over a widerange of processes andl pressures, employing a gas sealing system whichhas llong seal wear life, is inexpensive to repair and has the requiredlow leakage-into the process and to the atmosphere. In particular, theinvention enables loading of the seal to the exact desired amount withcontrolled fine adjustment to properly seat the seal, independent of theactual pressure of the process.

Another aspect of the invention is an extremely economical means insolid bowl separatorstfor sealing between the conveyer and bowltrunnions to prevent leakage and passage of solids between them, thusmaking it possible to use the solid bowl centrifuge design in highpressure processes with the resulting advantages of dryer solids andgreater emciency in many instances.

Yet another aspect of the invention is a means for eliminating thethrust effects of casing pressure upon horizontal overhung solid bowlcentrifuges, thus permitting the use of high speed radial ball mainbearings and avoiding the necessity of re-designing such centrifuges forpressure operation.

Yet another aspect of the invention-is the combination of features whichpermits the use of small, circumferential Contact primary gas seals atthe point where `seal-sare most likely to fail, thus making theirrenewal extremely less expensive than wouldv bev the case if the sealswere of the face Contact design.

We have realized that the. controlling consideration for producing asuccessful high pressure, universal centrifuge lies in the variablenature ofthe pressures which can occur in separator casings. The sealchamber has a seal at the casing which should be designed for a lowpressure drop, i.e., 5-15 p.s'.i., and if the pressure ydifferentialacross this seal is allowed to exceed the `design pressure by much, theseal life is drastically effected and excessive leakage P9 La occurs.Not only do diflerent processes require difleent pressure-s, but for anyprocess over an extended period of operation, pressures may not be keptconstant but may vary as much as from 1/2 to 2 times design pressure.The sealing chamber on the shaft at the point it extends from the casingmust contain butler gas at a higher pressure than the casing to preventsolids from entering the seal.

Vle have -solved this problem by regulating the -bul'er gas pressure bythe casing pressure, producing a constant pressure drop across the sealregardless of 'the actual value of the casing pres-sure. Then the sealcan be designed so that the gas forces act in a predictable manner andthe Yloading resulting from the dierential gas pressure can beminimized, which maximizes seal life.

-By making the pressure regulator adjustable, we have provided forslight adjustments in the field to initially properly seat the seals andaccommodate the different kinds of buffer gas that may be employed,since certain gases are more diicult to seal than others.

A further and very important aspect of the invention is our discoverythat by so regulating the buffer ga-s pressure, We are able to use aseal having a circumferential yarea primary seal surface. This type ofseal is presently mass produced for use in jet engines, for a usefullife of on the order of 500 hours in sealing oil bearings, and istherefore inexpensive. lts durability is extremely subject to pressuredrop. Despite its advantages of very low cost and small size relative tothe `so-called face seals, it has never found use in centrifugalseparators. We have found, however, that the foregoing regulation ofbuffer gas pressure enables its successful use.

This sealing system has particular' merit in combination with the solidbowl separator, with a second gas sealing chamber provided between thetwo shafts and with this chamber communicating with the outer one, sothat the pressure in both is regulated by the casing pres are.

Also, we have found that by the use of casing pressureregulated uidpressure applied to the outer end of the bowl trunnion, we can totallycompensate for the thru-st applied by the casing pressure whatever itsvalue may be, so that only high speed radial ball bearings need beemployed as the main bowl trunnion bearings, enabling the use of thesimple existinfr separator designs.

We have also provide a particularly efficient control system for fluidbalancing and sealing whether liquid or gas as well as a particularlyeihcient combination of buler gas system and seal design for smallcentrifugal separators.

The invention is illustrated by the preferred embodiment of a smallsolid-walled bowl, concentric trunnion, overhang type pressurizedcentrifuge.

ln the drawings:

FIG. 1 is a partially sectional and partially diagrammatic side view ofthe separator;

FIG. 2 is a side sectionalview, on a larger scale, of the sealconstruction employed by the machine of FIG. 1;

FG. 3 is a partially broken away perspective view, on a still furthermagnified scale of the circumferential seal- 4ing member incorporated inthe seal construction 0f FG. 2.

Referring to PIG. l, the centrifuge comprises a stationary housing l2,two concentric, horizontal axis rotating elements: a solid wall bowl i4and a hollow hub-screw conveyor lo with blade tips 17 shaped to litclosely to the contour of the solid bowl. High pressure process gas andfeed slurry are introduced within the conveyor hub through stationarypipe 18 and passes under the action of centrifugal force through theholes Ztl into the space between the conveyor and the bowl. he slurry isforced against the bowl, and the solids settle next to the wall. Theliquid flows outwardly at oneend over a Weir 22 and the conveyor lo,Vwhich is -caused to rotate at a speed slightly dierent than that of thebowl by the differential gear unit 24, causes the solids to move up theinclined portion atascos The bowl and conveyor are overhung in that theyare each supported from only one end,'the bowl ld being supported bybowl trnnnion 3d and the conveyer supported by conveyer trunnion 32mounted concentrically within the bowl trunnion. Two spaced-apart mainbearing pedestals 34 and 36 are provided, each supporting a high speedradial ball bearing 38, 39, respectively, which support the bowltrunnion. The housing for the planetary gear unit Z4 islixed to the bowlshaft between thevhigh speed bearings. The conveyor trunnionl whichturns slowly with respect to the bowl trunnion 3i? is supported relativeto the bowl trunnion by a pair of thrust bearings all and a bushing l5.

A flat drive belt 42 drives the ybowl trunnion 30 and the afxed gearunit 2d, and a planetary gear ratio Ycontrol shaft 4d controls thedifferential in speed between the bowl trunnion 3)- and the conveyortrunnion 32 that is produced by the gear unit.

r t the point where the trunnions 3% and 32 extend out of the casing l2,a sealing chamber 46 is provided, surrounding the bowl trunnion Si?. Asecond sealing chainber 48 is provided between'the bowl trunnion and theconveyer trunnion 32, and passages 5G com .iunicate between 14 ofthebowl to a level 26 spaced radially inwardly from f the weir 22, at whichpoint the bowl ends and the solids by centrifugal force are dischargedthrough outlet 2S.

the two seal clnrnibers.y A casing pressure conduit 5?, is connected tothe interior ofcasing l2, and extends to control a pressure relay deviceSe. The inlet conduit 55 of this relay is adapted to be connected to asource 5S of iuid, here an inert gas, at a steady pressure substantiallyabove the casing pressure, and the outlet from the relay comprises areferencepressureV conduit adapted to actuate the regulators to bedescribed. The relay Sais controlled by the casing pressure and reducesthe fluid pressure fror conduit Se to equal the casing pressure in lineS2, so that reference conduit d@ contains fluid that can be less noxiousand expensive than the casi g gas, but at the same pressure. Thereference press' re conduit d@ branches and connects to two pressureregulators o2. and 64. The regulator o?. is connected to inlet conduito5, which is adapted to be connected to a source of sealing gas at asteady pressure substantially in excess of the casing pressure. ThisYregulator discharges through conduit o to seal char lber do. Regulator62; is controlled by the reference pressure -in conduit o@ to produce inconduit @E5 a pressure greater by a small, steadytincrement over that ofthe casing pressure, regardless ofwhat the casing pessure is or how itfluctuates. The regulator o2 has handleo? which enables the adjustmentof the reduction ratio-of the regulator over a range of super-casingpressures to enable changing the loading of the seal chamber to adjustthe Contact of ealing surfaces, and to control leakage.

At the outer end 3d ofthe bowl shaft, a balancing chamber ed is defined,adapted to .contain a balancing fluid to exert inward pressure upon theend surfaces 7d, 72 and 7dof the bowl trunnion. ln this embodiment thetotal area of these end surfaces equals the sunrof the crosssectionalareas of therbowl and conveyer trunnions where they extend out of thepressurized casing, kat 7o, The seeond branch of the reference pressureconduit is connected lto regulator ed. The regulator has an'inletconduit "f3 adapted to be connected to a source of fluid Sii at a steadyVpressure substantially in excess of the casing pressure. `The regulatored is adapted todischarge through conduit S2 to the balancing charnber63.. The regulator 6d is controlled by the reference pressureof'conduitdll to produce in conduit S3 a pressure exactly the same as thereference and casing pressures'. Regulator 64' is preferably notadjustable Vto insure that the balancing 4effect cannot be disturbed. ti f i Referring now to FlGS. 2 and 3, the sealing chamber Viev/biebsurrounds the bowl trunnion 34) is defined on the trunnion side, by ahard cylindrical sleeve 33 which is Vsecured to the bowlk trunnion. v Alow pressure differential,

circumferential primary seal is effected between this sleeve andsealingring assembly at point The ring also makes-a secondaryy seal at with awall 9d, Vthe mating surfaces of face seal being` maintained in contactby spring 92. The wall 90 on oneV sideV is exposed to the casingpressure, and on the other side defines one end of the seal chamber. Theopposite end of the seal chamber is defined by a hard annular ring- 94sealed and secured to bowl trunnion 3f), and carbon sealing ring 96forming a face seal 98 with ring 9d. Sealing ring 96 is supported by aresiliently expansible bellows 19t) which is xed at its opposite end toextension 162 of wall gli. The expansible bellows defines with the othersurfaces a chamber which extends from the low pressure differential sealhaving primary sealing at 86 and secondaryV sealing at 88 to the highpressure differential primary seal 98. The sealing gas supply conduit 66is connected with this chamber and maintains the pressure therein at aslight constant pressure differential above the casing pressure.

Referring to PEG. 3, the low pressure differential seal ring assembly815 comprises three segmented carbon rings, seal ring d, back-up ring 1%and cover ring lill). Seal ring 10d has a cylindrical sealing darn 36'at the. portion of its cylindrical surface closest to the casing. Thisdam has an axial length of .02() inch. Buffer pressure slots 106 extendaxially from this darn across the entire remaining surface of ring lud,and bach-up ring i081. The cover ring llt) is mounted on the outerperiphery of the seal ring, and has the same axial length. A garterspring 112 having an accurately defined loadingy effect presses coverring and seal ring down upon the sleeve 33, causing the seal dam 36' toengage the sleeve 33 to form primary seal S6 with the exactly properforce. Garter spring 114 holds the back-up ring against the sleeve 33.The curved surfaces of both seal ring ftd-t and back-up ring 168 engagethe sleeve to position the seal, and to spread the loading effects ofthe springs, but the slots 1% cause the total pressure drop to occuracross dam S6. Ina known manner the narrowness of the dam S6 limits theeffective forces caused by the pressure differential. Most importantly,the regulator 62 insures that the pressure differential ofthecircumferential primary seal is kept at a substantially constantminimum, to enable the seal to run continously for the requiredthousands of hours without Wearing out.

The sealing ring assembly S4 forms the secondary -seal at 88 by means ofa second dam SS formed in the plane face of seal ring fli4l, againstwall 90. Pads 116 position the remaining portions of the seal ring andcover ring relative to wall 9u while allowing the seal pressure toextend to dam SSLthereby insuring that the pressure differential occursover only a short length of seal.

Referring back to FIG. 2, the high-pressure-differential sealing ring 96has a dam lfd positioned in the middle of the face of the ring andpassages lll) and 122 in conjunction with grooves lfl and l22 insurethat the total pressure drop occurs across this dam. This seal isdesigned for high pressure differential operation, so that a change inthe casing pressure, and a corresponding change in the pressure in theseal chamber results in a much smaller percentage change in loading onthis seal than in the low pressure differential seal, hence less drastican effect on wear.

Referring again to FIG. 1, the second seal chamber defined between theconveyer trunnion 32 and the bowl trunnion 3l) is formed by an O-ring E4at the casing end of the chamber, and a contact seal l26 at the otherend. An inexpensive pair of angular contact ball bearings 40 under thelow relative speed conditions between them can transfer thrust from theconveyer trunnion to the bowl trunnion.

While a very important advantage of the present invention is the factthat centrifugal separators can now successfully employlow-pressure-differential gas seals immediately adjacent the casings,where they are most likely to fail, and particularly ones havingcircumferential primary contact, with the attendant great advantages oflow-cost, small size, and ease of replacement, the invention also offerscertain advantages where the sealing fluid is a liquid.

d It should be understood that various of the specific details of theinventionv may be made within its spirit and scope.

Whatfis claimed is:

1. In a centrifugal separator comprising a pressure casing,

a solid-walled bowl within said casing,

a rotatably mounted bowl trunnion extending out of said casing, saidtrunnion being hollow and in driving relation with said bowl,

4a conveyer within said bowl,

a rotatably mounted conveyer trunnion extending concentrically withinsaid bowl trunnion, in driving relation With said conveyer,

meansoutside said casing fory driving said trunnions at high butdifferent speeds, said speeds being sufficient te produce highseparating forces in said bowl,

means for introducing into said casing and said bowl a slurry to beseparated and a gas to maintain superatmospheric pressure in saidcasing,

a first fluid sealing chamber surrounding said bowl trunnion at thepoint where it extends out of said casing,

and means for supplying said fluid chamber with seal fluid at a pressurehigher than the pressure within said casing,

the improvement comprising,

a low-pressure-differential fluid contact seal sealing between said bowltrunnion and said casing against leakage from said fluid sealing chamberinto said casing,

a high-pressure-differential fluid contact seal sealing between a Wallof said fluid sealing chamber and said bowl trunnion against leakagefrom said charnber to the atmosphere,

pressure regulating means for regulating the seal fluid pressure in saidfirst fluid sealing chamber,

control means for controlling said pressure regulating means, saidcontrol means connected and arranged in an exposed relation to saidcasing pressure, said control means connected to said regulating meansand adapted to operate said regula-ting means in response to said casingpressure, said pressure regulating meansV and said control meansconstructed and arranged to maintain the fluid seal pressure in saidfirst fluid sealing chamber at a constant positive pressure differentialabove said casing pressure,

a second fluid sealing chamber defined between said trunnions,

and means connected with said second fluid sealing chamber forsupplyingsaid chamber with seal fluid at a pressure higher than that within saidcasing.

2. The centrifugal separator of claim l wherein said means for supplyingsaid second chamber with seal fluid at a pressure higher than thatWithin said casing cornprise at least one fluid passage extending fromsaid second fluid sealing chamber through said bowl trunnion to saidfirst fluid sealing chamber, whereby the pressure in the two chamberscan be maintained the same.

3. The centrifugal separator of claim 1 wherein said pressure regulatingmeans comprise a pressure regulator, a first conduit communicatingbetween said pressure regulator and said means for supplying sealingfluid to said first fluid sealing chamber, and a second conduitcommunicating between said pressure regulator and said fluid sealingchamber, and said control means includes third conduit meanscommunicating with said pressure regulator and exposed to said casingpressure.

4. A centrifugal separator comprising,

a pressure casing,

a solid-walled bowl Within said casing,

a rotatably mounted bowl trunnion extending from one end of said bowlthrough said casing to the exterior of said casing, the other end ofsaid bowl terminating within said casing, said trunnion being in drivingrelation with said bowl,

means for driving said trunnion at a high speed to produce highseparating forces in said bowl,

means for introducing into said casing and said bowl a slurry to beseparated and a gas to maintain superatmospheric pressure in saidcasing, said pressure thereby producing an axial thrust against saidbowl trunnion relative to said casing,

sealing means sealing between said bowl trunnion and said casing,

`a balancing chamber enclosing a surace of said trunnion,'said surfaceextending at an angle to the axis of rotation of said trunnion in amanner to provide an effective piston area to receive an axial force,said surface directed away from said bowl,

means for introducing pressurized fluid into said balancing chamber toact upon said surface, the force of said pressurized fluid thereuponacting to oppose said axial thrust against said trunnion.

pressure regulating means for regulating the pressure in said balancingchamber,

control means for controlling said pressure regulating means, saidcontrol means connected and arranged in an exposed relation to saidcasing pressure, said control means connected to said regulating meansand adapted to operate saidregulating means in response to said casingpressure, said pressure regulating means and said control meansconstructed and arranged to maintain the tluid pressure in saidbalancing chamber at a level effective to balance said axial thrustagainst said trunnion.

5, The centrifugal separator of claim 4 wherein said bowl trunnion ishollow and said separator further comprises a conveyor within said bowi,

a rotatably mounted conveyor trunnion extending from one end of saidconveyor through said hollow bowl trunnion to the exterior of saidcasing, the other end of said conveyor terminating within said casing,said casing pressure thereby producing an axial thrust against saidconveyor trunnion relative to said casing,

means for driving said conveyor trunnion at a high speed slightlydifferent from that of said bowl trunnion,

and thrust bearing means located between said trunnions and adapted totransfer to said bowl trunnion said axial thrust produced by said casingpressure on said conveyer trunnion.

6. The centrifugal separator of claim 5 wherein said sealing meanscomprise 5^, Y trunnion at the point where it extends out of saidcasing, Y

means for supplying said iiuid sealing chamber with seal iluid at apressurefhigher tiran the pressure within said casing,

a low-pressure-diterential fluid contact seal sealing between said bowltrunnion and said casing against leakage from said fiuid sealing chamberinto said casing,

a high-pressure-dierential luid contact seal sealing between a wall ofsaid duid sealing chamber and said bowl trunnion against leakage fromsaid chamber to the atmosphere,

second pressure regulating meansV for regulating the seal uid pressurein said iirst iiuid sealing chamber, second control means forcontroliing said second pressure regulating means, said control meansconnectedand arranged in an exposed relation to said casing pressure,said control means connected to said regulating means and adapted tooperate Ysaid regulating means in response to `said casing pressure,said second pressure regulating means and said second control meansconstructed and arranged to maintain the seal fluid pressure in saidiirst huid seaiing chamber at a constant positive pressure diierentialabove said casing pressure, said separator further comprising a secondfluid sealing chamber deiined lbetween said trunnions, and meansconnected with said second uid sealing chainber for supplying saidchambery with seal fluid at a pressure higher than that within saidcasing.

seerences Cited by the Examiner UNITED STATES PATENTS 750,668 ,l/04Liedbeck.l Y 849,115 4/07 France Y 277-3 1,634,246 6/27 Jones et al233-13 2,095,587 6/35 Lorig 277-88 2,175,868 10/39 Bentley 277-32,496,471 2/50 Hornbostel Y 2,536,793 l/Sl Andersson et al. 233-l2,710,205 6/55 Brkich 277-3 2,835,514 5/58 McGahan 227---3 2,903,9709/59- Elovi'tz et a1. 227-3 X 2,917,228 12/59 Lewis et al. 233-12,956,824 10/60 Kuehler et al 277-32 X 3,051,497 8/62 Wigg et al. 277-33,069,173 12/62 Best 277-3 3,088,744 5/63 Ezekielet al. 277-3 HARRY B.THORNTON, Primary Examiner..

ROBERT F. BURNETT, Examiner.

1. IN A CENTRIFUGAL SEPARATOR COMPRISING A PRESSURE CASING, ASOLID-WALLED BOWL WITHIN SAID CASING, A ROTATABLY MOUNTED BOWL TRUNNIONEXTENDING OUT OF SAID CASING, SAID TRUNNION BEING HOLLOW AND IN DRIVINGRELATION WITH SAID BOWL, A CONVEYER WITHIN SAID BOWL, A ROTATABLYMOUNTED CONVEYER TRUNNION EXTENDING CONCENTRICALLY WITHIN SAID BOWLTRUNNION, IN DRIVING RELATION WITH SAID CONVEYER, MEANS OUTSIDE SAIDCASING FOR DRIVING SAID TRUNNIONS AT HIGH BUT DIFFERENT SPEEDS, SAIDSPEEDS BEING SUFFICIENT TO PRODUCE HIGH SEPARATING FORCES IN SAID BOWL,MEANS FOR INTRODUCING INTO SAID CASING AND SAID BOL A SLURRY TO BESEPARATED AND A GAS TO MAINTAIN SUPERATMOSPHERIC PRESSURE IN SAIDCASING, A FIRST FLUID SEALING CHAMBER SURROUNDING SAID BOWL TRUNNION ATTHE POINT WHERE IT EXTENDS OUT OF SAID CASING, AND MEANS FOR SUPPLYINGSAID FLUID CHAMBER WITH SEAL FLUID AT A PRESSURE HIGHER THAN THEPRESSURE WITHIN SAID CASING, THE IMPROEMENT COMPRISING, A LOW-PRESSUREDIFFERENTIAL FLUID CONTACT SEALING BETWEEN SAID BOWL TRUNNION AND SAIDCASING AGAINST LEAKAGE FROM SAID FLUID SEALING CHAMBER INTO SAID CASING,